Fossil fuels such as petroleum and natural gas which have so far been the most widely used fuels for motor vehicles and heating purposes are becoming scarce, are expensive and are not renewable. This is why alternative energy sources, which mainly use hydrogen as a transportable energy carrier in combination with a fuel cell, for example, are attracting more and more attention. It can therefore be assumed that hydrogen gas sensors will be needed on an increasingly large scale in both automotive and heating applications. Such hydrogen sensors can be used for determining concentration levels to allow optimized control of fuel cells as well as to fulfill alerting functions in situations where threshold levels must be detected. In order to ensure adequate safety, especially in monitoring applications, it is desirable that such hydrogen sensors are rugged and reliable and that they can be manufactured and miniaturized simply and inexpensively and be fitted, where possible, as an integrated unit.
The use of infrared gas sensors, which determine the presence and/or concentration of the gas in question by analyzing the absorption characteristics of the gas to be detected in a quite specific wavelength range in order to detect polar gases, such as methane or carbon dioxide, is known. Such gas sensors have a radiation source, an absorption path and a radiation detector. The radiant intensity measured by the radiation detector is a measure of the concentration of the absorbing gas; either a broadband radiation source can be used and the wavelength of interest can be adjusted via an interference filter or grate or a selective radiation source can be used, for instance a light-emitting diode or a laser in combination with light wavelength-selective radiation receivers. In the case of CO2, the characteristic wavelength in the infrared wavelength range is at 4.24 μm.
The detection of carbon dioxide is becoming increasingly important in the automotive field. This is firstly due to the fact that the CO2 content of the air in the interior of vehicles is monitored in order to increase the energy efficiency of heating and air conditioning systems so that, if necessary, i.e. if an increase in CO2 concentration occurs, a supply of fresh air is introduced via an appropriate fan flap. Secondly, modern air-conditioning systems are based on CO2 coolants and CO2 gas sensors can therefore fulfill a monitoring function in conjunction with issuing CO2 in the event of potential defects. In the automotive field in particular, sensors of this type must meet stringent requirements with respect to ruggedness, reliability and miniaturization and, at the same time, they must be extremely inexpensive to manufacture.